Airfoils of turbine blades and vanes of gas turbine engines often require a complex cooling scheme in which cooling air flows through the airfoil and is then discharged through carefully configured cooling holes, for example, cooling slots at the trailing edge of the airfoil. The performance of a turbine airfoil is directly related to the ability to provide uniform cooling of its external surfaces. Consequently, the control of cooling hole size and shape is critical in many turbine airfoil designs because the size and shape of the opening determine the amount of flow exiting a given opening, its distribution across the surface of the component, and the overall flow distribution within the cooling circuit that contains the opening. Other factors, such as backflow margin (the pressure delta between cooling air exiting the cooling holes and combustion gas impinging on the airfoil) are also affected by variations in opening size. In addition to conventional hole drilling techniques such as laser machining and electrical-discharge machining (EDM), complex advanced casting practices are typically used to yield airfoil castings with dimensionally correct openings in order to repeatably control opening size. Once cast, subsequent airfoil manufacturing operations must be performed such that cast-to-size openings are not processed through any operations that would significantly alter the dimensions of some or all of the openings.
Due to the severity of the operating environment of turbine airfoils, environmentally protective coatings are typically applied to these components when manufactured, and must be removed and reapplied during their repair. Diffusion aluminides and MCrAlY coatings overcoated with a diffused aluminide coating are widely used in the gas turbine engine industry as environmental coatings for airfoils. These coatings are produced by aluminizing the airfoil surfaces by such known methods as pack cementation, above-pack, chemical vapor deposition, slurry coating, and vapor (gas) phase deposition techniques. Each of these processes generally entails reacting the surfaces of the airfoil with an aluminum-containing composition to form two distinct zones, an outermost of which is an additive layer that contains the environmentally-resistant intermetallic phase MAl, where M is iron, nickel or cobalt, depending on the substrate material (e.g., mainly .beta.(NiAl) if the substrate is Ni-base). The chemistry of the additive layer can be altered with such as elements as chromium, silicon, platinum, palladium, rhodium, hafnium, yttrium and zirconium in order to modify the environmental properties of the coating. Beneath the additive layer is a diffusion zone comprised of various intermetallic and metastable phases that form during the coating reaction as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate. Diffusion zones in aluminide coatings on nickel-base superalloys typically include NiAl and secondary phases such as dispersed metallic phases (e.g., Cr, W), carbides (e.g., TaC) and sigma phases (e.g., CoCr), and a region of .gamma.--.gamma. structure that is locally depleted of solid solution strengthening elements.
As is apparent from the concerns discussed above regarding cooling hole dimensions, diffusion aluminide coatings must not prevent the airfoil from meeting numerous operational requirements, including airflow requirements for the cooling air required to flow through the internal cavities of the airfoil and exit through cooling holes and slots at the airfoil surface. As a result, cooling holes of new airfoils must be formed in an oversize condition in anticipation of the aluminide coating, or care must be taken to avoid or minimize aluminizing of the cooling holes. Likewise, aluminizing of airfoils returned for repair must also be performed with care to avoid or minimize aluminizing of the cooling holes and internal surfaces of the airfoils. Airfoils undergoing repair create additional process concerns, since the removal of a preexisting environmental coating and the subsequent application of a new coating can alter cooling hole dimensions. A typical approach for removing an existing aluminide coating is a combination of chemical stripping and grit blasting, which remove both the additive layer and the diffusion zone of the coating. To minimize impact on airflow control, internal surfaces and cooling holes, whether originally coated or intentionally left bare, are often masked, e.g., filled with wax, prior to coating removal in order to prevent removal of the aluminide coating from the walls of the cooling holes and internal surfaces. Such steps also prevent interaction of the stripping solution with oxides deposited within the cooling holes and on internal surfaces during engine operation. Otherwise, reactions between stripping solutions and oxide deposits can result in the liberation of oxide flakes that can plug cooling holes during subsequent engine operation, and/or result in entrapment of residual stripping solution in oxide scales, leading to rapid attack of airfoil internal surfaces when the blade is heated to elevated temperatures.
The preferred method for applying diffusion aluminide environmental coatings in the gas turbine industry has been by vapor phase deposition. Compared to pack cementation, vapor phase deposition produces a better surface finish, avoids hole plugging associated with pack powder, reduces heat-up and cool-down times during processing by eliminating a significant amount of the thermal mass, and improves coating cleanliness. Vapor phase processes include the use of "activators" such as fluorides, chlorides and bromides as part of the coating generation process. These activators are vaporized during vapor phase aluminizing, and freely travel through airfoil cooling holes and into airfoil internal cavities where, if the component being repaired has seen engine service, interactions occur with the oxide deposits and any aluminide coating remaining in the cooling holes and/or internal surfaces. These interactions are detrimental, leading to the reduction of oxides formed during engine operation and/or the deposition of new coating over existing coating and/or oxides. Coating deposited on top of an existing internal coating and/or internal surfaces that have an oxide scale previously formed during engine operation does not adhere well and can flake off during subsequent engine operation, leading to plugging of cooling holes and loss of cooling airflow control. Additionally, deposition of coating on internal surfaces can change pressure drops associated with movement of cooling air through complex internal passages of airfoils, leading to the deterioration of backflow margin. Masking solutions that can be employed during stripping of existing aluminide coatings are not appropriate during vapor phase aluminizing due to the elevated processing temperatures sustained.
In view of the above, it would be desirable if an improved method were available for protecting airfoil cooling holes and other internal surfaces during vapor phase deposition of a diffusion aluminide coating.